quartz for tx ?

[gallico58]

HelloI wanted to ask you if you can use a quartz-controlled circuit for a tx oscillator section of a metaldetector VLF ( idx-raptor-eccc....) for greater stability.
thank you

[Dave J.]

Quartz provides stability, but it means you can't easily do frequency shifting (for example to avoid a synchronous source of electrical interference).

[homefire]

Yes you can use Crystal Controlled Oscillator in the Tx section. There really is no Need. It may be beneficial in some cases of RFI or EMI holding Freq Stability lowering Jitter. Stability is rarely a issue in VLF detectors unless there is some type of Erratic Freq Changes or Serious Jitter. Any Minor Drift caused by Temp or what ever is not detrimental to the detector. The Receive function tracts the Tx oscillator as a Reference and really dose not care.

[gallico58]

Thanks for the answers
However sampling circuits will always work well, does not change anything for discrimination and the GEB using either oscillator free or quartzated at the same frequency?

[homefire]

As long as the Reference is not changing radically the Receive will track it only Seeing any Phase Changes as Targets and not just Chatter at you. Everyone Uses Colpit's or Even Hartley Oscillators because of low parts count.

[Davor]

With forced oscillation, have in mind that your coil's resonance must be quite close to the forced frequency, or otherwise you get off-resonance amplitude effects with proximity to the ground. To fix this, devices like Verator use Tx coil with low Q factor.
The other approach is to go completely square wave, and ditch resonance altogether, and employ some other method of energy recuperation. This may work even better than sinus oscillation, as the only amplitude noise source in Tx signal is power supply, which is usually lower than 1/f amplitude modulated noise provided by an oscillator transistor.

[holyuser]

Here is the resolution - Quartz stability Tx control, 2-tone sound and VDI display - All In One

[kosacid]

funny that i just started building one using a quartz to i have just built the tx part haven't even started on the amp stage

[Carl-NC]

Most modern designs use a micro-driven TX, not a free-running oscillator. Most SF-VLF are sinusoidal, all MF-VLF are square-wave drive.

[gallico58]

I'm experimenting with hef4060 and 560khz ceramic resonator to get after 8.75khz divisions followed by transistor buffer, 1khz for -5v circuit (2 transistors like tgsl), and 546hz use for audio section.
You would save at least 2 integrated.
Tried with sampler style raptor seems to be fine, tx sine 16v disc signals and geb ok to oscilloscope, i go ahead


Hello and thanks

[Dave J.]

That's a good use of a 4060. Another thing I've seen done is a free-running LC oscillator driving a CMOS division chain to derive audio and a charge pump negative supply.

For audio, use a pulse width and total period that are integer multiples of the transmit period. That way the audio spectrum has a null at the transmit frequency and won't crosstalk to the receiver.

[F117]

Most SF-VLF are sinusoidal, all MF-VLF are square-wave drive.

To drive a TX coil from eg a PIC microcontroller, do you need a full bridge FET setup, switching the coil between Gnd and Vcc, or can the coil be switched ON/OFF with a single logic gate FET - like a PI coil ?
Are there any examples of such a square wave TX configuration ?

Thanks,
- Bernard

[Davor]

As Carl said, all multifrequency detectors have it. Say CZ - it has half bridge. Recuperation is incredibly simple - just put large enough electrolyte capacitor as a sort of bridge decoupling, and that's it. When polarity changes the coil will be charging the capacitor (or a battery), and gradually it will change and start sucking it, but then comes a next polarity change and so on. With half-decent MOSFETs, low ESR electrolytes, and a coil with sensible Q factor, this setup can be more energy-efficient than a sinus.
From Rx point of view, it is basically all the same.
My next build, the one I'll do from the scratch, will surely have a square-wave Tx. It is all benefits and no drawbacks. With quiet power rails it will outperform a sinus oscillator in every imaginable department.

[Carl-NC]

To drive a TX coil from eg a PIC microcontroller, do you need a full bridge FET setup, switching the coil between Gnd and Vcc, or can the coil be switched ON/OFF with a single logic gate FET - like a PI coil ?
Are there any examples of such a square wave TX configuration ?

For MF you need a half-bridge (CZ, DFX) or full-bridge (BBS/FBS). For SF sinusoid a single transistor will work.

[F117]

Thanks, Carl & Davor